The increase in the number of mobile devices subscribing to Long Term Evolution (LTE) type wireless communication systems has created high cell loads, both in terms of resource block (RB) utilization (e.g., >75%) and the number of simultaneously connected or on-line devices in a cell. Mobile users are expected to enjoy a wide-range of mobile applications generating diverse types of “smart phone traffic”, with a spread of data packet sizes and inter-packet arrival times. These conditions are favorable for achieving large gains through the use of multi-user diversity and frequency selective scheduling (FSS), thereby providing an improved smart phone experience to their users.
When the number of mobile devices in a cell is large, it is not practical to give rich channel state information (CSI) signaling to every on-line mobile device to achieve good FSS. Rich CSI signaling means detailed sub-band CSI across the entire carrier bandwidth with a relatively short update period. This can result in large signaling overhead, and may exhaust all available sounding reference symbol (SRS) resources due to repeated transmission of SRS on each sub-band by all on-line mobile devices. High cell loads make it difficult to manage SRS transmission and allocate sounding resources to mobile devices in a way that maximizes important performance metrics such as cell uplink throughput or uplink throughput of mobile devices located near the edge of a cell. Rich CSI is essential for accurate FSS. Knowledge of the frequency response across the whole bandwidth allows identification of the best frequency bands for each mobile device.
Frequency specific CSI should remain accurate across multiple scheduling periods, such as 1 millisecond in LTE, before it is refreshed with another sounding. Frequent CSI measurement is necessary even for slow moving mobile devices. For example, sounding the whole band every 20 ms (with individual sub-band sounding transmissions every 5 ms) is recommended for pedestrian users moving at about 3 km/h. For higher mobile user speeds, the reporting interval should drop in inverse proportion to the speed. Rich sounding comes at a cost. For example, in some conventional networks every sub-frame may be configured to use one SC-FDMA symbol as a SRS, resulting in a sounding overhead of 1/12th or about 8%. It is therefore desirable to configure a mobile device for rich sounding only when necessary. The sounding configuration, or SRS reporting configuration, of a mobile device may be changed through the use of a radio resource control (RRC) connection reconfiguration procedure. While, the resource block cost of a RRC connection reconfiguration is low, it does consume a number of physical downlink control channel (PDCCH) resources. There is also a risk of encountering a radio link failure and dropping a call when a mobile device is encountering poor radio conditions such as when the mobile device is at the edge of a cell. Because of this, it is recommended to keep the RRC connection reconfigurations low, such as less than one hertz per mobile device.
Thus, there is a need for methods and apparatus for assigning sounding resources to mobile devices that effectively maximize desired performance criteria, such as throughput of cell edge users, while at the same time keeping RRC connection reconfigurations to a minimum.